Field of view and object size

I’ve been curious how big things are and what they’ll look like in my telescope. Before learning about astronomy I thought it was all about looking at stars which are always point size and boring. I didn’t realize there were all these nebulae and galaxies and planets that have apparent size!

I tried a couple of calculators for telescopes and came up with these stats for my stock Orion 8″ Dob with the stock 25mm Plössl eyepiece:

  • Magnification: 48x
  • Field of view: 1.08° = 3900″
  • Exit pupil: 4.2mm
  • Theoretical resolving power: 0.57″

Adding a 2x Barlow doubles the magnification and halves the field of view (exactly?). Changing the focal length of the eyepiece makes a linear change in magnification and FOV; a 12.5mm eyepiece would have a 96x magnification and 0.54° FOV. Theoretical resolving power is purely a function of the scope, not the eyepiece, so doesn’t change. I’ve ordered a 7.5mm eyepiece and a 2x Barlow; if I stack those I’ll get 320x magnification and 0.2°  = 700″ FOV.

Now then, how big are things? Some approximate values of point sources from Wikipedia. All numbers are arcseconds

  • Moon: 1800″
  • Jupiter: 30–50″
  • Saturn: 15–21″
  • Betelgeuse: 0.05–0.06″

Betelgeuse is interesting: it’s a very large and nearby star so its apparent size is pretty big, second biggest from earth. It was the first star to have its diameter measured, via interferometry. I’m sad that even with my 320x magnification Jupiter and Saturn are still going to appear pretty small; at best Jupiter will fill 7% of my view.

And how big are non-point sources? Some numbers from a table of Messier Objects:

  • Andromeda Galaxy: 6500″ x 3700″
  • Pleiades: 6000″
  • Orion Nebula: 4000″ x 3600″
  • Ring Nebula: 70″ x 60″

Now I get why the Ring Nebula was so hard to see; it’s only as wide as 2% of my scope’s view. The Pleaides are 1.5x the size of my scope view, which explains why it’s so hard for me to get oriented to them. And Orion just about fits, although the part I can actually see with my eyes is smaller.

I should make a point of finding the Andromeda Galaxy.

I took the scope out last night around 9:30pm with no particular plan. Went back to Jupiter, eye-searingly bright and with the moons all neatly spaced out with lovely symmetry. And the Orion nebula, now easy for me to find and understand. And the Pleiades, which I still don’t really get. Also tried just sort of randomly pointing at stuff which was absolutely unrewarding. Heh.

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